This invention relates to the measurement of the electromagnetic radiation emissivity of surfaces in the infrared wavelength spectrum. It relates specifically to apparatus and a method for determining the emissivity of camouflage material such as woven or nonwoven fabrics or plastic sheets or nets, slightly above ambient temperature. The apparatus and method of this invention are particularly useful as a practical production line system wherein a particular value or range of values of emissivity is likely to be specified for camouflage material. It provides a means of monitoring the emissivity of camouflage material for quality control during its manufacuture on a production line.
Blackbodies radiate electromagnetic energy in accordance with the Stefan-Boltzmann radiation law, the rate of energy emission being proportional to the fourth power of the absolute temperature of the radiating blackbody. Nonblack bodies radiate electromagnetic energy at a lower rate than do blackbodies at the same temperature, the ratio of nonblack to blackbody radiation at any given temperature being the nonblack body's emissivity.
Radiometers measure the radiant energy emitted by a radiating source. They are calibrated to indicate the temperature of a blackbody which would emit the observed thermal radiation received by a radiometer detector. This temperature will herein be referred to as the apparent temperature. The difference between the true physical temperature of an emitting surface and its apparent temperature as measured by a radiometer affords a means of determining the emissivity of the emitting surface.
Methods and apparatus for the determination of the electromagnetic emissivity of materials are known. U.S. Pat. No. 4,645,358 describes a method and apparatus for determining the electromagnetic emissivity of antennas in the wavelength range of interest in radiocommunications and apparatus involving a radiometer. It involves the determination of the apparent temperature of a test object at two different physical temperature levels and an algorithm for calculating emissivity from the two apparent temperatures and the two actual temperatures. It requires the use of a cryogenically cooled surface serving as a stabilizing source of uncorrelated radiation in the system by radiating at a constant temperature far below that of the system, thereby keeping uncorrelated-radiation at a minimum.
U.S. Pat. Nos. 4,172,383 and 4,465,382 describe systems in which a test surface is irradiated with blackbody radiation from a furnace at two different energy levels, and the energies reflected by the surface are measured with a radiometer. An algorithm and the components for a computation system for continuously computing the emissivity of the test surface are provided.
U.S. Pat. No. 4,408,878 describes system comprising two radiometers, one in a substantially totally reflecting enclosure and the other in a substantially totally absorbing enclosure, which are moved to the immediate proximity of the surface whose emissivity is to be determined, and then removed, thereby providing signals from which the emissivity of the surface can be calculated.
Although the apparatus and methods taught by these patents allow the determination of emissivity of objects and employ, in some instances, apparatus components similar to those used in the present invention, e.g. radiometers, none of the apparatus and methods described lend themselves to continuous, on-line monitoring of emissivity near ambient temperature as envisioned by the present invention.